This invention generally relates to removal of particles from a liquid system and more particularly to a thermophoretic method and system for removing particles included in a particulate containing liquid.
The removal of small particles from chemical process fluids is a general problem to which a wide variety of techniques have been applied. Liquids are much more difficult to filter than gases for several physical reasons. For example, Brownian motion causes aerosol particles to travel much farther in gases than in liquids, providing an increased opportunity for the particles to collide with and stick to a membrane. In addition, viscous drag is much smaller in gases (for the same volume of material) so that smaller pore sized filters can be used in gases without unduly limiting pressure drops. Furthermore, the adhesion of particles to, for example, a membrane is less likely than in gases due to larger viscous drag forces present in liquids thereby making it more likely that particles are more likely to be re-entrained in the liquid.
Most filters for liquids rely on sieving, using pores having dimensions smaller than the particles to be captures; this leads to more clogging problems than occur with filters for gases. Moreover, the strong forces exerted by liquid flow impingement on the filters can destroy the filters. In addition, poor initial wetting of filter membranes can reduce the sieving performance of liquid filters. Other shortcomings of membrane filters include the limited ability to recycle the membranes for use in different processes. In view of the several shortcomings of sieving (filtering) to remove particles from liquid processes, other techniques have been developed which have provided partial solutions.
For example, electrophoretic techniques have been developed using dipole fields in molecular separations. Multipole fields are useful for filtering liquids having sufficiently low electrical conductivity. Uniform electric fields can be used to remove charged particles from high-resistivity (low conductivity) liquids. A general difficulty with using electric fields is that the magnitude and sign of the charge on a particle, if any, depends upon the pH of the solution, the type of particle and the type of solution. Further, any particles without an electrical charge will not be removed. These limitations restrict the applicability of filtering using electrophoresis techniques.
Another example includes magnetophoretic techniques which apply inhomogeneous magnetic fields to exert variable forces on particles suspended in solution depending on a particle""s magnetic susceptibility. Although magnetophoresis is a useful tool in separation of materials in the field of waste re-cycling, it appears inefficient for applications requiring ultra-small particle separation.
Yet another technique that has been developed to remove particles from a liquid solution is a flotation mechanism or settling rate. Generally, depending on the particle size and particle charging characteristics determining particle adhesion characteristics, the particles including adhering groups of particles display certain settling behaviors. Generally, larger groups of particles settle faster than smaller groups. This method, however, is generally inefficient in terms of processing time and is limited to larger particles that settle within a reasonable time.
One area where there has been limited application of the method to the problem of liquid filtering includes thermophoresis. Thermophoresis works by the presence of differential pressures exerted on particles due to the presence of a thermal gradient, causing the particles to migrate from an area of higher temperature to an area of lower temperature. The phenomenon of thermophoresis is well known and has been observed in gases, for example, aerosols, and liquids some time. For example, one application of thermophoresis relates to particles dispersed in a fluid trapped between two plates of differing temperatures will migrate towards and eventually deposit on the colder plate. This application, however, is not practical for use as a particle liquid since deposition on the surface of a flow cell or flow cavity is transitory, turbulence or pressure fluctuations being sufficient in many cases to agitate the particles back into the liquid flow stream.
Therefore, there is a need to develop a thermophoretic system and method whereby a wide range of particle sizes present in a liquid may be effectively captured and removed from the liquid without creating an unacceptable pressure drop while providing for cost effective recycling of the particle filtering means.
It is therefore an object of the invention to provide thermophoretic system and method whereby a wide range of particle sizes present in a liquid may be effectively captured and removed from the liquid without creating an unacceptable pressure drop while providing for cost effective recycling of the particle filtering means, while overcoming other shortcomings and deficiencies in the prior art.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method and apparatus for thermophoretically removing particles from a particulate containing liquid.
In a first embodiment according to the present invention, a method is provided for thermophoretically removing particles from a particulate containing liquid including providing a heated turbulent flowing particulate contain liquid through a first conduit; redirecting a portion of the particulate containing liquid through a second conduit to provide laminar flow having a flow direction substantially parallel to the first conduit; forming a thermal gradient in said second conduit substantially perpendicular to the flow direction; concentrating particles in the particulate containing liquid in a portion of the second conduit aided at least in part by thermophoretic forces; and, separating the particulate containing liquid into at least a relatively concentrated particle containing portion and a relatively unconcentrated particle containing portion.
In a related embodiment the thermal gradient is formed by thermally conductively contacting said upper portion of the second conduit with the lower portion of said first conduit the thermal gradient having a decreasing temperature profile in a direction toward the lower portion of the second conduit.
In another related embodiment, the step of separating the particulate containing liquid further comprises collecting the relatively concentrated particle containing portion in a lower portion of the second conduit such that the relatively unconcentrated particle containing portion is collected in an upper portion of the second conduit exiting through an upper exit portion of the second conduit to be returned to the first conduit the lower portion exiting through a lower exit portion of the second conduit.
In another embodiment, thermal conductance in a direction other than substantially perpendicular to the flow direction is minimized.
In yet another embodiment, the method further includes maintaining the temperature gradient by providing a heat sink at a relatively lower temperature than the first conduit and the second conduit said heat sink in thermally conductive communication with the second conduit.
In another embodiment, the method further includes the step of removing particles from the particulate containing liquid included in the lower portion exiting through the lower exit portion of the second conduit.
In a related embodiment, the step of removing particles includes passing the particulate containing liquid over at least one depositing surface to form a flow pathway such that particles included in the particulate containing liquid are captured and entrapped upon impacting said at least one depositing surface.
In another related embodiment, the at least one depositing surface is in thermally conductive communication with a heat sink maintained at a temperature relatively lower than the at least one depositing surface temperature.
In yet another related embodiment, the at least one depositing surface includes a plurality of protrusions extending into the flow pathway to form a plurality of particle capturing spaces.
In a separate embodiment, the steps are performed in parallel according to a plurality of serially connected first and second conduits.
In a separate embodiment according to the present invention, a thermophoretic system is provided for removing particles from a particulate containing liquid including a first conduit with means for generating a turbulent flow therein said first conduit having a second conduit formed substantially parallel to a flow direction thereto; said second conduit being in bypass flowable communication with the first conduit such that a portion of the turbulent flow is redirected to the second conduit to form a laminar flow in the second conduit; the second conduit being in thermal contact with the first conduit for forming a thermal gradient substantially perpendicular to the flow direction to exert a thermophoretic force on particles included in the laminar flow.
In a related embodiment, an upper portion of the second conduit forms thermally conductive contact with the lower portion of said first conduit thereby forming the thermal gradient said thermal gradient having a decreasing temperature profile in a direction toward the lower portion of the second conduit.
In another embodiment, the second conduit further includes an inlet flowable pathway and at least an upper outlet flowable pathway and a lower outlet flowable pathway said upper outlet flowable pathway in flowable communication with the first conduit to return a portion of the laminar flow to the first conduit. Further, the thermophoretic system further includes a means for selectively regulating a flow through the second conduit to maintain laminar flow conditions.
In another embodiment, thermal conductance in a direction other than substantially perpendicular to the flow direction is minimized by providing thermally insulating plumbing connections.
In another embodiment, the thermophoretic system further includes a heat sink in thermally conductive communication with the second conduit for maintaining the thermal gradient.
In a separate embodiment, the thermophoretic system further includes a particle removing means in flowable communication with the lower outlet flowable pathway. Further, the particle removing means includes a plurality of depositing surfaces disposed along a flow pathway such that particles included in a particulate containing liquid are captured and entrapped upon impacting said depositing surfaces. Further, the plurality of depositing surfaces are in thermally conductive communication with a heat sink maintained at a temperature relatively lower than the depositing surfaces. Further yet, the plurality of depositing surfaces include a plurality of protrusions extending from the depositing surfaces into the flow pathway to form a plurality of particle capturing spaces.
In a separate embodiment, a plurality of thermophoretic systems are disposed in serial flowable communication to increase a particle removing efficiency.
These and other embodiments, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures.